Add API for setting/getting current locale

This API is used to set the current locale for libobs, which it will set
for all modules when a module is loaded or specifically when the locale
is manually changed.

test/win/test.cpp

@@ -71,7 +71,7 @@ static void CreateOBS(HWND hwnd)
 	RECT rc;
 	GetClientRect(hwnd, &rc);
 
-	if (!obs_startup())
+	if (!obs_startup("en"))
 		throw "Couldn't create OBS";
 
 	struct obs_video_info ovi;